For more than 30 years, performance car enthusiasts wondered if a stock 1964 Pontiac GTO could really stack up against a Ferrari 250 GTO, or if it could really run 13-second quarter miles, as road tested by Car and Driver when new. And it turned out that one GTO in particular – with a big secret under its hood – could, and that particular GTO will make an appearance later this year at one of the many events celebrating the 50th anniversary of the Goat.

Available as an option package on Tempest LeMans models (in order to get around a corporate ban on installing engines more than 300 330 cubic inches in intermediate-size models), the GTO borrowed its name from the Ferrari GTO, although Pontiac had no plans to homologate the car with the FIA for road racing. When Car and Driver road-tested the Pontiac GTO for the March 1964 issue, its cover boldly proclaimed “GTO vs. GTO” and showed a Pontiac GTO trailing a Ferrari GTO by a car length on a road course. Inside the pages, however, the coverage was limited to the domestic GTO, as the magazine had difficulty locating a Ferrari GTO to borrow for comparative purposes. Based on data in its files, Car and Driver declared that, “The Pontiac will beat the Ferrari in a drag race, and the Ferrari will go around any American road circuit faster than the stock Tempest GTO.” Performance aside, the Pontiac version sold for a fraction of its Italian namesake, and was therefore of significant interest to readers of more modest income.

In the text, Car and Driver admitted that the GTO models tested were not off-the-shelf cars; instead, it proclaimed that the two specimens tested were delivered through performance-oriented dealership Royal Pontiac, and equipped with the “Bobcat package” to improve both performance and driveability. Specific changes to the Tri-Power 389-cu.in. V-8s included a change in main jets to .069 on all three carburetors (versus .066 on center carburetor and .073 on outboard carburetors in stock form), for improved acceleration; a progressive action throttle linkage, for smoother off-idle performance; modified distributor advance, which improved low-end torque but required high-octane fuel; thinner head gaskets, a blocked heat riser and new spark plugs (without gaskets) to raise compression; and unique rocker arm retaining locknuts, which allowed the engine’s hydraulic lifters to function as solid lifters. Compared to the output of a standard Tri-Power 389 (348 horsepower and 428 pound-feet of torque) the Bobcat package was said to add somewhere north of 30 horsepower, but the trade-off was that high-octane gasoline had to be used to avoid knock.

But there was more to it than that. Perhaps the first clue that the GTO tested by Car and Driver at the drag strip was more than mildly tuned came when the magazine complied its testing numbers. The sprint from 0-60 MPH took a mere 4.6 seconds, while the run through the quarter mile went by in 13.1 12.8 seconds at a trap speed of 115 112 MPH when the car was fitted with drag slicks. As Car and Driver was quick to point out, that was faster through the quarter-mile than a stock 289 Shelby Cobra, and also faster than Ferrari’s own GTO. It was also quite a bit quicker than other road tests of the day had achieved with the Pontiac GTO: Car Life, for example, reported a 0-60 MPH time of 6.6 seconds, with the quarter-mile going by in 14.8 seconds at 99 MPH, while Hot Rod’s first test of the GTO was more succinct, calling the car “a slug.”

Manifest for the Ringer GTO. Photo courtesy Carlisle Events.

It was that Hot Rod review that fired up Jim Wangers, who reportedly demanded that chief engineer John DeLorean allow him to select the cars used for future road tests. One such car was a red 1964 Tempest LeMans, special ordered by Wangers to include the GTO package; a Tri-Power 389; a close-ratio four-speed manual transmission; 3.90:1 Saf-T-Track rear differential; transistorized ignition; and “insulation delete” to save weight. Upon completion, Wangers returned the car to Pontiac and requested that a 421-cu.in. Super Duty HO Tri-Power V-8 be installed; as this engine was essentially a bored and stroked 389, few visual cues existed to alert observers to the switch, particularly after Wangers had the 421’s identifying transfer lug removed. Once this was accomplished, the car was sent to Royal Pontiac for the Bobcat treatment, then shipped to Car and Driver‘s David E. Davis for testing. Though no specific horsepower figures were reported, a Super Duty an HO Tri-Power 421-cu.in. V-8 would have produced 370 horsepower in 1964, and the Royal Bobcat package likely would have brought the engine’s output to somewhere north of 400 horsepower.

At the time, Wangers would only admit that the car given to Car and Driver for drag strip testing was “well set up.” Speculation that the GTO tested was a ringer began almost immediately, but the car-buying public didn’t care. In the GTO’s first year on the market, 32,450 copies were sold, while for 1965 this number more than doubled, increasing to 75,352 (and jumping to 96,946 for 1966). The so-called “Ringer” GTO had created the exact buzz that Pontiac needed, and the strength of the Car and Driver review helped propel the Pontiac GTO to its iconic muscle car status. It wasn’t until 1998 that Wangers came clean about installing the 421 in the GTO.

Now owned by director, producer, and writer Tenney Fairchild, the Ringer GTO was recently restored by Scott Tiemann of Super Car Specialties. Believed to be the only Pontiac GTO fitted with the 421-cu.in. V-8 by Pontiac itself, the car represents a significant piece of Pontiac, GM, and muscle car history, and will be displayed for the duration of this year’s GM Nationals show at Carlisle.

The Carlisle GM Nationals will run from June 20-22. For additional details, visit CarlisleEvents.com.

I’m certain any miffed GTO owners will finds buyers today at full MSRP for cars in showroom condition if they feel in any way deceived!!! Love the skullduggery, Jimbo! Like it was all cake and violins in the auto media game back then…. (Or now, for that matter.)

In 64 I was a dedicated foreign car fan who worshiped Ferraris and the Car and Driver story was total heresy. Then a couple of Weeks later I saw my first Pontiac GTO, a gold two-door sedan; filling the night air with blue smoke as it performed an incredible burnout down a suburban Kansas City street. Suddenly doubts about the inherent superiority of the Maranello products began to form. We were still arguing about that article five years later when I was in Vietnam. It’s nice to know that, like most of us, the Goat that started it all has survived.

Dave, I among many others really are thankful for your service my friend.That tire smoke you saw and that brute horsepower got to you as you realized it was damn near affordable! My first ride in a ’64 in about 65 with a well tuned car and great driver did me in at the early age of 13!

Talk about a trip back in time – I remember that road test. We young’uns were a bit miffed that C&D couldn’t come up with a REAL Ferrari GTO [probably not a problem today] for an actual side-by-side [or front-to-rear, as we College Boys were convinced that the Pontiac would be an ill-handling P-I-G.]

Royal Pontiac! ANOTHER trip back in time! The “skunkworks” of its day.

Of course, GTO eventually worked its way into our DNA, and, altho I was more partial to sports cars [after I graduated and found a REAL job], I hope “making an appearance” at Carlisle includes a trip to the strip. Cue Ronnie and the Daytonas:

Neat car, and I well remember that road test. I kept that magazine around until some time in the mid 1970s when we moved and my wife cleaned out my many year collection of Car and Driver and Road & Track.

However the article contains an oft repeated misstatement. The 421 was not “just a bored and stroked 389″. You cannot safely bore a 389 enough to do it. The 421/428 (bored 421)/455 while still the basic Pontiac block were cast as different engines internally. They have larger main bearings and the SDs were cast with provisions for dry sump oiling, although to my knowledge that was never actually factory installed except perhaps on some early factory drag cars. They were also cast with larger bores, allowing the 421 to be bored to 428 and the 428 to be stroked to 455. The engine put in the “ringer” was a factory 421 SD, available only over the counter through parts as a crate engine at the time. The 370 hp was also a joke, even at the time. The normal 421 was rated at 370 with tri-power (conservative), but the SD, with its 2x4bbl set-up was rated at 410, and no one believed that. The NASCAR engines were factored at 410 with a single 4bbl. One of TV reality car shows recently did a vintage rebuild of one (2x4bbl.) and achieved 488hp (flywheel)on the dyno!

CAPTTOMB, mea culpa. After checking with our own in-house Pontiac expert, it turns out that you, and not my original reference source, were correct. I’ve since updated the piece to reflect this, and I thank you for setting the record straight!

The comparison got me so PO, I actually wrote and got published in their letters section. They
edited my letter. I actually asked if their next comparison would be of the Pontiac GP to a Ferrari GP.
Hey! What do you expect from a 14 year old?

Fate has a way of coming, full circle. When I was 20 I bought a red 1966 recovered theft from a junkyard. It needed tri-carbs, 4 speed and an interior. After I put it together it sold for $1200.00 at Manheim Auctions in New Jersey.
Later that summer I learned that I could bolt 421 heads on the 389 block, for a buddy’s ’67. He said car was a beast with the change, too bad I didn’t really know what I was doing? All I saw was larger valves and 2 heads I wanted to get rid of…

I have 2 distinct memories of the GTO’s of that era I rememebr My Dad was using a friends 65 GTO convertible it was one of the coolest cars out their it was gold with light color interior. He had all three of my other brothers and sister besides me in the car we were heading out to a Amusement Park were my Father had a construction project going on down the street. My younger brother enjoyed the convertible so much he kept standing on the floor so the wind could be in his face he was too young to be tall enough to have his head barly over the height of the back seat Dad kept telling him to sit back down at one point he got behind Dads seat we kept quiet he ended up falling aselep standing up. Dad found out he was not happy by today standards we would be in trouble. The other was we had a new Priest join the parish at the School I was attending he came from a wealthy family he never took a paycheck while he was at our parish. He comes pulling in to the School parking lot with a brand new 1967 GTO of course we all thought he was the coolest Priest around….He ended up being just that good person all the way around….

Agreed, he is, now. Google his image when the 1964 GTO debuted. 50 years changes the way people look. He was middle aged then. The ’64 GTO still looks fine, but you and I probably look far different than our yearbook photos now. Bill might be referring to the top red GTO photo. That’s Mr Wangers behind the steering wheel recently.

I was 15 and regularly read Car + Driver. I would ride my bike or in one of our interminable winters…walk down to either one of our local drugstores and buy my monthly copy of the magazine.

All except that edition. It sold out before I could get my young enthusiast hands on it . I never did get to read the GTO vs GTO test, until the 1990’s when I bought a Brooklands Performance Pontiac series book that had this article.

A great article and the kind of article that separated C+D from other magazines of that era. Although I also liked Car Life, R+T, Hot Rod and Cycle World.

For some reason we never did get any ’64 …or possibly ’65 GTO’s in town. Back then of course Canadian Pontiacs were essentially Chevy engined and full sized. No 389’s or 431’s….just 230 sixes , 283, 327 and 427 V8’s.

The hot Canadian Pontiac intermediate was a Pontiac badged Acadian of the SS 396 Chevelle….think it was called the SD 396….or maybe just SD.

First time I saw a GTO was when I was in Chicago in 1964. Also first time I saw a 409.

Lesmore…..sounds like we had much the same childhood. I lived in Saint John, New Brunswick and there were hardly any fancy cars (lunch bucket town). I don’t believe the GTO came to Canada until maybe ’68 or possibly it was 1970 (along with the rest of the Tempest line). I think there were a couple of 409’s in town – one ’62 Belair and a ’64 Pontiac convertible.
Barry Thomas’ “Wheel to Wheel” blog

By that I mean ordered by a Canadian, living in BC…shipped up here through GM Canada from Michigan ?

Through the ’50’s and ’60’s we didn’t have any American Pontiacs sold in Canada. By American Pontiac, I mean Pontiacs…with wide track, the Pontiac V8…the earlier flathead Pontiac straight eight or straight six.

I remember the first American Pontiacs to hit the Canadian market. The ’68 GTO’s and the ’67 Firebirds. No Chevy engines, just Pontiac engines. Even though I think Trans Am race rules allowed Firebirds to use Chevy small blocks….none to my knowledge came that way.

All Pontiacs sold here, had Chevy engines.

However Canadian market Oldsmobile, Buick and Cadillac all were the same as American market models sold in the states.

GMC trucks however didn’t use the American engines (305 V6, Pontiac V8’s)…they were sold with Chevy engines only…in Canada.

Even the 409 valve covers for the Canadian market didn’t have the Chevy Bowtie stamped. They were all plain valve covers. I’ve got a couple of Chevy truck Canadian market 409 valve covers that are bereft of the bowtie.

The GTO was an awesome car in 1964/65. I’ll never forget the sound and sight of one going through first and second just smoking those red line tires. Back then 400 + cubic inches seemed scary! Had to be a 4 speed!

Many of us in that era were concerned only in straight line performance. The vast majority of my crowd was happy with this mentality, although in other driving applications, our cars would handle like a hippo on roller skates at seemingly the most inopportune times.

Hmmmm… I thought the limitation was 330 cubic inches (wish I had my Pontiac books with me right now). And anyway, the 389 is less than 400. Later, GM had a limit of ten pounds per horsepower, which is why they would quote different hp numbers for Firebirds versus GTOs, with the same engine. Usually it involved a tab on the carb linkage to prevent the carb from opening all the way.

Not to get off track here, but I recall a cool 1967 LaMans that my mother-in-law bought new that had a 6cyl 4-barrel aluminum heads and I also believe, a aluminum block. It would put you into the seat on a rolling start, but off the line, a little sluggish. I tell this story to many but they don’t believe me. I am a “MOPAR ” fan, but do enjoy any cool cars. This being one.

well,i have a similar story. in 1997 the company I worked for was going to get me a new D6r. now at the time the R was the newest dozer in that line. the dealer delivered a dozer,but it wasn’t mine. I ran it a week or so and it was great. turned easy,power,easy on fuel and the newer one’s now rolled easy when placed in neutral.the hitch was this was a ”demo”,not to be sold,only used as a sales tool. why ? ‘cuz I think the factory ”blueprinted this machine and spent time on it to sell more cats. a ringer if you will. when my dozer came it was nowhere near what the ringer was. if only we knew then.

Aweome testiment to definition of muscle car! BUT note 4 very very rare actual 421 Royal Bobats were built. Two perished in fires in late ’60s. Reminaing are: this one in article and the only other one also red with an automatic Super T 300. Was in museum in south Florida; and recently displayed at GM museum in Mich.

All said. My ’64 GTO was a lot of fun and in hindsight a great waste. A 389 automatic pillar coupe that I campainged in oval track racing and destroyed it. A friend of mine airbrushed (on the trunk lid) the picture of a hand coming out of the clouds, gripping my little blue goat on the track, with the caption “GTO, God Takes Over”

The first GTO in my town was a beige ’64 sedan with window posts and brown interior. The kid in my high school who owned it would thunder by my house, usually kept in about one gear lower than necessary for the posted speed limit. Just amazing sound from that supposed compact economy car.
The beige color really was awful but sure made it a sleeper.

I always admired (secretly, since I had a Ford) the 1st GTO in my hometown. It was a 64 “post” as we called them back then, and it put on some great tire-spinning shows. He quickly quit racing us on the street though, because he always spun so much, most of us could beat him in our necessarily short drag races. I know, it was only the driver, but it sure made me smile when I beat him out of a Drive-in with my Falcon Sprint in front of a lot of the guys, ha ! If we had raced a few seconds more, he would have blew me away ! Take ’em as you get em’, right ? I got a 66 with a later model 455 with tri-power and 4 speed years later…what a torque monster ! Great article, thanks.

Three out of three I had the hemi. It was all because the Hemi driver just floored it….and then sat there….and sat there…barely moving forward. Meanwhile my 327 was running through the gears quite handily, way down the strip.

When I usually tell this story, I ‘forget’ to let listeners know that it was a ‘traction’ issue….lol.

The GTO’s may not have come with a 421 but the 389 Tri-Power cars did not disappoint. They were faster than any other Detroit street car with the exception of the 64 Fuelie Vettes – and most of those races were decided by driver skill – not horsepower. The GTO always delivered on the promise of performance and deserves the “legendary’ ranking.

I was hooked on cars as a very young boy before that article was published (it was the first car magazine I ever purchased with my meager allowance and I STILL have it!) but it made me a lifelong Pontiac fan. I owned a ’64 GTO convertible and have an ’06 GTO (yes, I know, it’s really a Holden but still it goes really fast like a GTO should) and I have an all original ’72 GTO, which I consider the last of the “real” GTO’s, since after that they became big “bumpered” or ever worse, Novas. RIP, little GTO. I’ll go to Carlisle just to see this car!

Around the Olds dealership I worked at back then, GTO stood for Go To Oldsmobile. About the cubic inch limit, my understanding was always 400 cid in an A-body. Perhaps not in ’64 but it had to be so in 1965; the Buick GS got away with a 401 as an existing engine. Those were definitely great times and great cars.

I have loved cars, especially muscle cars as far back as I can remember. When I was a teenager, I drooled over GTOs and other cool cars but had to make do with a ’69 Pontiac Firebird as my first project car and a couple years later a ’65 Corvair Corsa that I rebuilt with my dad as my first car. But I always wanted a GTO. Years later, I bought my first GTO, a ’67 hardtop 4 speed HO car. I had several years of fun driving it all over the place as I restored it to a nice looking driver. Then one year the tax man wiped me out and I had to sell my beloved GTO. As soon as the car was sold, I took the little money I had left and bought a ’68 GTO hardtop. I spent the next 4 years restoring that car from a tired ugly looking car to a beautiful car I could be proud of. After 11 years of using it as a weekend car (and occasional back up car when my regular car was down), my ex-wife painted me into a financial corner where I had to sell it. As luck would have it, a good friend passed away a couple years later (around the time I said good-bye to the ex-wife) and his family wanted me to have his beautiful and mostly original ’67 GTO hardtop (also a 4 speed and HO, like my first GTO), but this car still has only 47k original miles on it, still has original paint, interior, vinyl top, drivetrain (except engine) and has been and continues to be a garage kept beauty. There is nothing like taking my GTO out for a drive on one of the local country roads and hearing that engine “howl” when I open the secondaries on the Quadrajet. I’m lucky that my friend had my current GTO signed by Jim Wangers years ago. My car is probably a “ringer” as well, given the addition of a modern hotter than stock cam, ’70 vintage disc brakes from another GTO and 15 x 7 Rally II wheels from a ’71 Firebird (to clear the disc brakes). It goes faster, corners better, and stops faster than it did originally. Even so, it is a time machine that takes me back in time to my teenage years and puts a big grin on my face.

Too the bad Pontiac line is no longer with us. Pontiac was responsible numerous styling and performance innovations that later were copied by other mfrs. It seems the only times there was a concept that later became a legend, it had to be back-doored around GM senior management. These executives were primarily focused on counting beans, and not thinking out of the box. This led to late “market catch up” cars like Camaro, 442, Gran Sport, Firebird and others.

We have a ’64 Lemans convertible that is staying a Lemans!
I think there are over 60,000 ’64 GTO’s out there now… lol
Ours is getting a complete restify,. gone is the 326ci 2 bbl… gone is the Super Turbine 300… gone is the 2.73 pegleg rear!
’71 HO 455 .030 over, 700r4 w/3.55 posi.
Hopefully will be back on the road this winter down in Phoenix!
Every GTO owner I have talked to appreciates us staying with the Lemans.
BTW my favorite year GTO is ’68
My Dad had a ’66 tri power 4-speed in the early ’70s… sold it one year before I started to drive!
Got my Moms ’66 Bonneville instead, wish we had it now!

First, very glad that you are staying with the LeMans without adding GTO stuff. I’m basically a Ford guy by nature(meaning some guys have blue eyes, some have brown, and that’s just the way they’re wired, really no difference), but I had a couple of encounters with LeMans. The first is that one of my secondary school pals’s parents owned a new 64 LeMans convertible (white with black interior) that I never drove but rode in, and I always liked the design inside and out. The other is that about ten years after I left home, my mom bought a 67 LeMans hardtop. I did drive that while visiting her in LA, thought it was comfortable and liked the bucket seats and console interior, but think it could have used quicker steering. She would have kept that car years longer, but it was rear-ended a few years later and replaced with a Gran Torino Elite!!!(what a boat). As the woman in When Harry Met Sally said( and I’ll adapt it), I’ll have what you already have, the 64 LeMans. That was reinforced in me when I picked up a used copy of the 1964 NY Auto Show program, and bound into it was the complete special brochure promoting just the GTO. It’s always just one man’s opinion, but I think the 64 was the cleanest design of the 1960’s Pontiac intermediate cars.

My first new car (fresh out of college and a new job as a high school teacher) was a 65 GTO, silver with black vinyl roof and interior. I ordered it from a dealer to include 4-speed, Tri-Power and rallye wheels. Six weeks later, due to a strike at GM, it finally came in. When I rushed over to pick it up, there were no rallye wheels and only a 4-barrel carb. When I started complaining, the sales rep assured me that he could quickly sell it to at least a dozen others if I didn’t want it. I took it, signed the papers, and rolled out. I put over two hundred miles on it that Saturday afternoon. Monday morning, I was the coolest teacher in school. My only regret was that I decided not to order AC. Had to cut back somewhere. My salary was $450 per month. The note on the car was $110. Still have fond memories of that car.

I think it is important to note the difference in curb weight between the 3 chassis options. It is my understanding that the post coupe is significantly lighter than the hardtop and the convertible. Also, how an individual car is set up is as important as driver skill when real world performance is the issue. I have an all original ’65 convertible GTO, which I have had for 25 years. After a quarter of a century driving this car, I have learned a thing or two about how to drive it well. It is a factory 6 bbl, 4 speed car, stripped down and geared low. On a certified truck scale, it weighs all of 3440 lbs, with 5 gallons in the tank, the spare in the trunk, and the registration in the glovebox. Traction can be a HUGE problem, but if you are smart, you will address the control arm bushings in the rear end. Replace the stock rubber bushings with graphite impregnated polyurethane bushings and use adjustable shocks. Don’t forget soft compound tires in the rear, and experiment with tire pressure to find out what the car likes. With the wheel hop cured and the wheel spin under control, the car can hook up nicely when launched correctly. That is, with 3 grand held on the clock, fed just enough clutch to get going without bogging the engine, and once underway, dump the clutch and nail the throttle. Super tune the engine, paying close attention to the overall balance of the fuel and the spark in relation to the cylinders, that is, consistency in rocker arm ratios and spark energy. If you have access to race gas, limit the centrifugal advance to about 14 degrees BTDC, and crank up the initial advance to about 20-22 degrees BTDC, for a total advance of about 34-36 degrees BTDC. I am being honest when I tell you that low 13’s to high 12’s are possible, depending on driver skill. Select your rear tire height to have the engine red line in 4th gear as you are going through the lights, about 110 MPH for me. The combination of minimal weight, a high state of tune, optimal gearing, and skilled driving is a winner, even if the car is stock. There is a world of difference between a 4 bbl, 2 spd. auto with a 3.08 gear, loaded with comfort and convenience options(READ-WEIGHT), and a stripped down, tri-power, 4 spd. clutch, with a 3.90 gear. The former is a slug, the latter is a BEAST! My buddy with the LS1 Camaro is still scratching his head on how to beat the old goat. One more point to mention: Don’t drive the car like a Chevy with a mouse. Winding the mill to 6 grand is hard on parts, and will garner slower e.t.’s. It is better to shift sooner and take advantage of all that wonderful torque. AND, if you can, degree the camshaft to optimize the valve timing. I installed my cam a few degrees advanced and saw a big difference from being installed straight up. Great, This One.
I love the Garbage Truck Option, and I can understand why Ed Cole hated this car so much. With the right set up, it is death to any version of “Uncle Glass” of similar vintage. That includes the rat motored variants. Qualified.
I have heard it said that any number of GM execs have said that that car (the GTO), changed every rule in the corporation. Gee, That’s Odd.

Wow! All that work and you’re not quite as quick as my 289 powered ’67 Cougar! I’ve done about the same work to mine as you have: gears, tires, tuning, one carb, a 625 cfm Carter AFB. Also solid lifter cam and with ported factory heads it runs 12.8 ET’s. Weight is 3400 with me in it. And it does it with 100 fewer cubic inches! That’s FORD power for you!!

Lord, crazy ’bout a mercury!
Very nice. I’m particularly keen on the Comets.
I’m curious as to what your combination is.
It doesn’t sound very stock.
Are those heads Westlake Gurney?
I’m not ported or cammed, mind you.
Let’s have it, now!

OK, here are the details for the 12.8 sec. times. Stock rear suspension with the addition of slapper bars. Nitto Drag Radial tires. 4.56 gears. Wide ratio Top Loader 4 speed. Engine has original block, crank and 7 rods (dropped a valve awhile back and split a cyl. wall and put the valve head through a piston so replaced the rod, too). Crank is still std. size journals. Cam is a generic aftermarket solid lifter, 236 deg. duration, .530″ gross lift, purchased over 25 years ago so is old school compared to the new grinds. Valve train has aluminum roller rockers and screw-in studs. The intake is an Edelbrock Air Gap dual plane. The carb is a Carter AFB 625 cfm, also over 25 years old. I rebuilt the engine about 25 years ago with flat top pistons ,040″ oversize and home ported ’65 289 2 barrel heads. Actual displacement works out to 294. The best it ran was in the 12.90’s, usually low 13’s.. After I dropped the valve I went to ’97 Explorer factory heads, again home ported with factory size valves. Headers are 1 5/8″ long tubes. This combo got consistent 12.8’s and is what I compared to your GTO.
As a footnote, it broke a pushrod last year and chewed up a few parts so I “upgraded” with some eBay stuff: used dome pistons and a set of “bargain” aluminum heads which were a close-out from Holley. It took a lot of rework to get these heads useable! I had been going cheap on valve springs which was the cause of my failures so went to beehive springs. Still tuning n this setup; best so far is 12.74 and 109.76 MPH. I live in Flagstaff, AZ and have to drive to Phoenix to run so don’t get to go as much as I’d like. I recently retired so may get to run more, but now can’t afford fancy parts. So except for head work, it’s all bolt-on. Oh, I launch around 5,000 RPM and shift at 7200-7400. No torque in this little engine!